INTEGRAL and RXTE/ASM Observations on IGR J17098-3628

To further probe the possible nature of the unidentified source IGR J17098-3628, we have carried out a detailed analysis of its long-term time variability, as monitored by Rossi X-ray Timing Explorer/All Sky Monitor (RXTE/ASM), and of its hard X-ray properties, as observed by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). INTEGRAL has monitored this sky region for years, and significantly detected IGR J17098-3628 only when the source was in its dubbed active state. In particular, at ≥20keV, Imager on Board of the INTEGRAL Satellite/INTEGRAL Soft Gamma-Ray Imager (IBIS/ISGRI) caught an outburst in 2005 March, lasting for ∼5 d with a detection significance of 73σ (20--40 keV) and with emission at <200 keV. ASM observations have revealed that the outburst of the soft X-ray lightcurve is similar to that detected by INTEGRAL; however the peak of the soft X-ray lightcurve lags behind, or is preceded by, the hard X-ray (>20 keV) outburst by ∼2 d. This resembles the behavior of X-ray novae like XN 1124-683, and hence further suggests a Low-Mass X-ray Binary (LMXB) nature for IGR J17098-3628. While the quality of the ASM data prevents us from drawing any definite conclusions, these discoveries are important clues that, coupled with future observations, will help us to resolve the nature of IGR J17098-3628 that is unknown so far.